Light scattered by smoke.
How does smoke send light to a detector outside the beam?
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How does smoke send light to a detector outside the beam?
Fit emitter, off-axis photocell and chamber
A dark labyrinth admits air while blocking outside light. In clear air the emitter misses the photocell. Smoke particles scatter some light sideways into the photocell and raise its signal.
Illustrative optical signal: 8% scattering collection, 0.4 m³/mg extinction factor, 0.1 mW detector threshold. It demonstrates geometry, not certified smoke-detector sensitivity. Heat is not the trigger. The unshielded-emitter signal is a worst-case direct-light teaching case, not a quantitative ray-trace; the photocell stays off the main beam.
Trigger the alarm with scattering smoke while a clear chamber stays quiet.
Distinguish smoke scattering from direct-light false alarms.